To measure rim width:
This is most easily accomplished if the tire is dismounted from the rim. Measure from the
inside tire seat to the outside tire seat. Do not measure the width between the two ridges as this will often be very inaccurate.

If this is not an option due to tool considerations then you can read the tire, you will find a code similar to: "P205/40R16". The "R16" part is the rim width.
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R16 is rim width? hahaha

"Which do I prefer? Chess or sex? It depends on the position." - Spassky